review classifier: every ACP run gets one shared, empty cwd

opencode writes the session's cwd into the system message's environment
block ("Working directory: …", "Workspace root folder: …"), ahead of the
PR's user message. Each ACP classification ran with its per-call scratch
directory, /tmp/review-classifier-<pid>-<counter>, as cwd, so every call
sent a different system message and the provider's prompt cache could not
match past it.

The session's cwd is now one fixed directory, <tmp>/review-classifier-acp-cwd,
shared by every run. HOME (the config, system.md and opencode's own state)
stays a directory per run, removed after it, as before. With no tool and
no project config nothing writes into the cwd: a live opencode 1.15.10 run
through initialize, session/new and a prompt left it empty, with its state
all under HOME. So concurrent runs can share it, and no state carries from
one run to the next.

Refs #5245

swarm-review-classifier/src/main.rs

@@ -70,6 +70,7 @@ async fn serve() -> Result<()> {
             deadline: swarm_queue_client::review_classifier::CLASSIFICATION_DEADLINE,
             cancel_wait: acp::CANCEL_WAIT,
             cost: cost::Otel,
+            cwd: std::env::temp_dir().join("review-classifier-acp-cwd"),
         },
     };
     let secret = client_secret_with_retry(&client_id).await?;